Just a few weeks before Election Day in Washington state this year, polls showed voters were solidly behind a measure calling for labels on genetically modified food. Proponents, from state farming coalitions to D.C.-based advocacy groups, felt optimistic.

But then came a surprise, although it followed what seems to be an emerging pattern. When voters cast their ballots, the measure went down, handing the pro-label camp a defeat — though a narrow one. Only 38,000 votes separated labeling supporters from victory, out of 1.75 million.

The same thing happened in California last year. Polls showed voters backing a similar state measure. But when the votes were tallied, it also lost, 53 percent to 47 percent.

With these narrow losses and the costly, high-profile campaigns that led to them, the fight over GMO labeling has hit the national stage two years in a row, giving labeling advocates a sense of momentum — and opponents new battlegrounds.

It may include Congress. While lawmakers haven’t demonstrated an appetite for acting on GMO labels, at least one food industry group that fought the Washington state proposition has begun talking to members.

At the same time, some labeling efforts have gotten traction in Congress. This year, Sen. Barbara Boxer, D-Calif., introduced a labeling bill that has 14 co-sponsors. A companion bill on the House side, introduced by Rep. Peter A. DeFazio, D-Ore., has 48 co-sponsors.

Advocates see victory as inevitable. “We’re already beginning to lay the groundwork for labeling fights in more than 20 states in 2014,” said Scott Faber, head of Just Label It, a national pro-GMO-labeling campaign. “This really is an issue where we’ll ultimately prevail. It’s just a matter of when.”

Not, however, if the industry has anything to say about it. In the months before the California and Washington state measures went before voters, the biotechnology industry, led by Monsanto and DuPont, along with the food and beverage industry, including big names such as Coca-Cola and Nestle, dumped millions of dollars into defeating them. Their campaigns aired ads saying the measures were confusing, had too many exemptions — for restaurants and alcohol, for example — or that food costs would rise.

The messages resonated. Amid noise from both sides, voters were ultimately convinced that California’s proposal was sloppily written and would make food manufacturers vulnerable to lawsuits. In Washington, advocates blamed record-low turnout, especially among younger voters, for the loss.

In either case, the polls were deceiving: When voters are asked if they want more information on food packaging, they say yes. But when they’re told they might have to pay for it, they balk.

For advocates, the debate is easily framed as a right-to-know issue. Why should Americans not know if genetically modified ingredients are in their foods when 64 other countries require such labels?

“[The Food and Drug Administration] requires 4,000 additives and processes to be labeled. High fructose corn syrup is labeled. So are other sweeteners,” said Colin O’Neil, director of governmental affairs for the Center for Food Safety. “Why not distinguish between corn and GMO corn? It’s inconsistent.”

The industry faces bigger challenges in trying to frame the debate. For starters, the technology is difficult to grasp and easily vilified. Also, the industry is seen by critics as trying to hide something — and it clearly has something concrete to lose if GMO labels become mandatory.

In the European Union, where labels are required, food manufacturers just reformulate products with more expensive, non-GMO ingredients because a GMO label in that market would be tantamount to a skull and crossbones. Last summer, Monsanto announced it was pulling its products from the EU approvals process, essentially conceding it doesn’t want to cope with the popular and regulatory resistance there.

But on home turf, the industry’s putting up a fight. In California, the biotech and food companies spent $46 million, and in Washington state $22 million. Roughly half of the Washington figure came from food companies via the Grocery Manufacturers Association, the food industry trade group, although that was only revealed after the state’s attorney general sued the association, saying it was violating state law by not disclosing contributors.

When the association eventually disclosed its funding, some notable companies were absent from the list, including Mars, Unilever and Kraft. Those businesses spent $3 million in California, but bowed out of the Washington battle, something critics took as an additional sign that companies didn’t want to risk the public relations hit.

In documents submitted to the state after the lawsuit was filed, the association’s strategy was clear. In an email, the association’s board wrote to its CEO, saying it would direct “GMA staff to seek Federal Preemption with No Mandatory Label.” On the heels of the Washington state win, the GMA said it was talking to lawmakers about federal legislation that would codify labeling standards.

“A 50-state patchwork of GMO labeling laws would mislead consumers,” the group said in an email recently, when asked about its strategy. “Americans should be protected nationwide with consistent regulation of food safety and food labels — determined by our nation’s most qualified food safety expert — FDA. This is the position we have shared with state legislatures, city councils and members of Congress.”

The FDA has long held the position that GMO ingredients are “substantially equivalent” to conventionally produced foods, and therefore don’t require a label. But the concept of GMO ingredients — now in an estimated 70 to 80 percent of processed foods — leaves some people uneasy despite an abundance of regulatory and scientific assurances declaring them safe.

In 2001, the agency issued draft guidelines saying companies could choose to label GMO-containing products or declare if a product contains no GMO ingredients. Since then, sales have soared of both third-party-certified, non-GMO products, and organics, which by law can’t have GMO ingredients.

While the FDA still hasn’t issued its final guidelines, the industry argues that consumers who want non-GMO food clearly have options. Critics note that no company has elected to indicate on packaging that a product contains a GMO ingredient. The grocery industry plans to petition the FDA in 2014, asking it to allow foods with GMOs to be labeled “natural.”

The biotechnology industry, meanwhile, has embarked on an information offensive. Last week, Biotechnology Industry Organization, the major trade group for agriculture biotechnology companies, trumpeted a new initiative, called GMO Answers, launched in July. The program aims to take questions from the public online in an attempt to demystify GMOs. “We realized it was time to change the conversation,” said Cathleen Enright, executive vice president with BIO. According to Ketchum, the PR firm that runs the site, the most frequently asked questions are about labeling.

Enright underscored the industry’s concern that labeling efforts were undermining consumer confidence in GMO technologies. If that happens to the extent it has in Europe, the industry says research-and-development budgets will shrink and important potential technologies — for example disease- or drought-resistant crops — won’t ever reach the public.

Still, despite the battles, it’s not clear what impact labels would have in the U.S. marketplace. “Very few consumers are reading the labels up and down,” Faber conceded. “Most consumers aren’t going to turn around every package to look for a genetically engineered ingredient label.”

At the state level, Connecticut and Maine earlier this year passed GMO labeling laws, although they’re contingent on neighboring states doing the same, and 24 other states considered similar measures.

“No one thinks this patchwork of laws makes sense, but in the absence of federal leadership, we’re taking it to the state level,” Faber said. “This fight will intensify, state by state. A number of states will acquire [GMO] labeling, and the focus will turn to Washington, D.C.”